1. Field of the Invention
The present invention relates to a stop changing device for a camera, and particularly to a stop changing device favorable to a single-use camera of a lens-fitted photo film unit and so forth.
2. Description of the Related Art
As a sort of simplified cameras, lens-fitted photo film units of various types are put in the market by the present assignee. The lens-fitted photo film unit includes a simple photographing mechanism including a taking lens, a shutter mechanism, and so forth. Further, the lens-fitted photo film unit is loaded with an unexposed photographic film in advance. A user can easily take a picture having good image quality. As to the lens-fitted photo film unit, f-number and shutter speed of the taking lens are fixed for the purpose of lowering its price and reducing its size so that exposure conditions are constant. Thus, a range of a photographic condition for properly photographing a main subject and the background thereof is narrow. If subject brightness is outside the above-noted range, the photographic image quality deteriorates. Meanwhile, some of the lens-fitted photo film units have a built-in flash unit. In virtue of the flash unit, it is possible to take a picture in a state that the subject brightness is low, namely in a room, in the night-time, and so forth.
With the recent innovation of technology, it is required to improve the performance of the lens-fitted photo film unit. In view of this, it is considered to incorporate a mechanism, which changes the exposure conditions in accordance with various photographic circumstances, into the lens-fitted photo film unit. Japanese Utility Model Laid-Open Publication No. 1-130118 discloses a lens-fitted photo film unit provided with a stop changing member, which changes a stop by setting a movable stop plate to a photographic optical axis. The movable stop plate is formed with an aperture smaller than a fixed stop aperture. The stop changing member is associated with a flash switch operated for selecting whether or not a flash device emits a flash light. When the flash switch is turned on, the flash light is emitted and the fixed stop aperture is used to focus on a nearby subject. When the flash switch is turned off, emission of the flash light is prohibited. At the same time, the smaller stop aperture is used to make a depth of field deep. Owing to this, it is possible to properly take a picture not only of a nearby subject but also of a middle-distance subject.
In the meantime, Japanese Patent Laid-Open Publication No. 9-5817 discloses another stop changing device provided with a movable stop plate, a photometry unit, and an electromagnet. The movable stop plate is formed with an aperture, which is smaller than a fixed stop aperture. The photometry unit measures subject brightness, and outputs a setting signal when a light amount is a predetermined value or more. The electromagnet is activated when the setting signal is outputted. In this stop changing device, the movable stop plate is associated with a winding operation of a photographic film to approach the electromagnet. When the electromagnet is activated, magnetic suction is caused between the movable stop plate and the electromagnet to hold the movable stop plate against a spring. When the electromagnet is not activated, the magnetic suction is not caused between the movable stop plate and the electromagnet so that the movable stop plate is drawn by the spring.
Regarding the lens-fitted photo film unit in which setting of the stop is performed in two steps so as to be associated with the foregoing flash switch, an overexposure is caused when flash photographing is performed under conditions of the sunshine daylight and the larger stop aperture. Hence, product quality deteriorates. Moreover, printing time is prolonged in a laboratory so that printing cost increases. Further, since the depth of field becomes shallow, the background of the main subject is out of focus. The blur of the background is conspicuous in the daytime photographing, because the background is photographed brightly. Meanwhile, when a picture is taken at an obscurity place, for example at the evening outdoors, many of photographers judge that the flash light is unnecessary. Thereupon, photographing is performed in a state that the flash switch is turned off. Accordingly, the smaller stop aperture is used for photographing so that it is likely to cause an underexposure.
As to the lens-fitted photo film unit in which the stop is set to either of two states in accordance with a result of photometry, a diameter of the smaller stop aperture is determined such that the overexposure is not caused at a high-intensity value (+5.0 EV) of film latitude when photographing is performed at an upper limit of general brightness. It is desirable to change the stop on the basis of a proper exposure amount of the smaller stop aperture. In a case the photographic film is a negative photosensitive material, the optimum exposure amount is +1.0 EV. On account of this, the stop is changed when the subject brightness is substantially EV12.0. Photographers, however, are likely to judge that it is still bright when the subject brightness is within a range of EV12.0 to EV10.0. For this reason, photographing is performed without emitting the flash light so that the underexposure is caused.
In view of the foregoing, it is a primary object of the present invention to provide a stop changing device for a camera in which exposure is automatically and simply controlled in accordance with subject brightness and flash photographing.
It is a second object of the present invention to provide a stop changing device for a camera in which an exposure amount is adjusted to prevent an underexposure from occurring in a dark place.
In order to achieve the above and other objects, the stop changing device for the camera comprises a single stop plate, which is movable relative to a photographic optical axis. The stop plate is formed with a first stop aperture and a second stop aperture. The first stop aperture is smaller than a fixed stop aperture, and the second stop aperture is smaller than the first stop aperture. When subject brightness measured by a photometry unit is a predetermined level or more, the stop plate is moved so as to position the second stop aperture at the optical axis regardless of an operation of a flash switch. When the subject brightness is less than the predetermined level, the stop plate is moved so as to position either of the fixed stop aperture and the first stop aperture at the optical axis in accordance with the operation of the flash switch. Concretely, the fixed stop aperture is used at the time of flash photographing, and the first stop aperture is used at the time of non-flash photographing.
In a preferred embodiment, the stop plate is rotatable and the stop changing device includes an electromagnet and a stopper besides the rotatable stop plate. The electromagnet is activated to hold the stop plate against a spring. Thereby, the second stop aperture is positioned at the optical axis. The stopper enters a movement area of the stop plate in accordance with the operation of the flash switch. When the non-flash photographing is performed, the stopper enters the movement area of the stop plate to abut on the stop plate. Owing to this, the first stop aperture is positioned at the optical axis. When the flash photographing is performed, the stopper is evacuated so that the stop plate is allowed to move to the outside of the optical axis.
In another embodiment, the stop plate is slidable and further has a large opening formed between the first and second stop apertures. The large opening is used for setting the fixed stop aperture. In this embodiment, the stop changing device includes an electromagnet, a first stopper, and a second stopper. When the subject brightness is the predetermined level or more, the electromagnet holds the stop plate against a spring so as to position the second stop aperture at the optical axis. The first stopper enters the movement area of the stop plate in association with the operation of the flash switch. When the flash photographing is performed, the first stopper enters the movement area of the stop plate to abut thereon. Owing to this, the large opening is positioned at the optical axis. When the non-flash photographing is performed, the first stopper is evacuated to allow the further movement of the stop plate. The second stopper abuts on the stop plate, which has slid across the first stopper, to position the first stop aperture at the optical axis.
According to the present invention, a structure of the stop changing device may be simplified and the manufacture cost thereof may be reduced. Further, it is possible to adjust an exposure amount without enlarging a size of the stop changing device so that a picture may be taken at a dark place without causing underexposing.